Intermetallic layers and coatings are often formed on a surface of a metal component to protect the underlying metal substrate of the component and to extend its useful life during operation. For example, many superalloy components in gas turbine engines, like turbine blades, vanes, and nozzle guides, include an aluminide coating on airflow surfaces that protects the underlying superalloy base metal from high temperature oxidation and corrosion. Among other applications, gas turbine engines are used as aircraft or jet engines, like turbofans, in electromotive power generation equipment to generate electricity, such as industrial gas turbine engines, and as power plants providing motive forces to propel vehicles.
Generally, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel, such as jet fuel or natural gas, and igniting the mixture, and a turbine blade assembly for producing power. In particular, gas turbine engines operate by drawing air into the front of the engine. The air is then compressed, mixed with fuel, and combusted. Hot exhaust gases from the combusted mixture pass through a turbine, which causes the turbine to spin and thereby powers the compressor. Aircraft gas turbine engines, referred to herein as jet engines, propel the attached aircraft forward in response to the thrust provided by the flow of the hot exhaust gases from the gas turbine engine. Rotation of the turbine in industrial gas turbine engines generates electrical power and motive power for vehicles.
Air flow surfaces of certain gas turbine engine components are directly contacted by the hot exhaust gases. The hot exhaust gases heat these components to high temperatures and expose them to impurity elements, like sulfur, originating from the combusted fuel. Superalloys, in particular, are susceptible to severe oxidation and corrosion in such harsh environments, particularly when the superalloy components of the gas turbine engine are exposed to the hot exhaust gas stream created in a jet engine. One type of corrosion results from enhanced oxidation experienced by superalloys at high temperatures, such as the portions of the gas turbine engine directly exposed to the hot exhaust gas stream. Sulfidation is another type of corrosion experienced by superalloy gas turbine engine components exposed to sulfur originating from the hot exhaust gases and other environmental sources. Generally, sulfidation increases the oxidation efficiency of superalloys and, in particular, the oxidation rate of nickel-based superalloys. Sulfidation is often observed in superalloy gas turbine components that are heated to temperatures below about 1500° F. when directly exposed to exhaust gas steams. Sulfidation also occurs in superalloy gas turbine components having portions that are shielded from exposure to the direct exhaust gas stream and, as a result, operate at a temperature less than about 1500° F. For example, certain gas turbine blades include an airfoil segment that is heated to a temperature greater than 1500° F. when exposed to an exhaust gas stream, a root used to secure the gas turbine blade to a turbine disk of the gas turbine engine, and a platform that separates the airfoil segment from the root. In such gas turbine blades, the root, which is not directly exposed to the exhaust gas stream, is heated by conduction from the airfoil segment and also cooled to less than 1500° F. by heat transfer to the more massive turbine disk.
To shield gas turbine components from hot exhaust gases, a ceramic thermal barrier coating may be applied directly to the superalloy substrate is an addition to an aluminide coating. As a result, the combustion and exhaust gases from the gas turbine engine may be hotter than would otherwise be possible with only a protective coating of aluminide. Increasing the temperature of the hot exhaust gases improves the efficiency of operation of the gas turbine engine. However, such ceramic thermal barrier coatings may not adhere well when applied directly to the superalloys commonly used to form gas turbine engine components and, while in service in the gas turbine engine, tend to spall.
To improve adhesion and thereby spalling resistance, a bond coating may be applied to the gas turbine engine component before the ceramic thermal barrier coating is applied. Intermetallic aluminides, like platinum aluminide and MCrAlY's, are common examples of such bond coatings that have been in use for many years. However, platinum aluminides are expensive to produce, which contributes to increasing the cost of gas turbine engine components and the cost of refurbishing used gas turbine engine components. MCrAlY's must be applied using expensive equipment.
Accordingly, there is a need for a gas turbine engine component with an aluminide coating that improves on conventional aluminide coatings and methods of forming such coatings on gas turbine engine components. There is also a need for a gas turbine engine component with a bond coating that is competitive in performance with platinum aluminide and less expensive to produce than platinum aluminide, and for methods of forming such coatings on gas turbine engine components. There is also a need for a gas turbine engine component with a modified aluminide coating that can protect the coated area specifically from sulfidation, and methods of forming such coatings on gas turbine engine components.